Evaluation Electronics of a coriolis mass flow sensor

Abstract

In the evaluation electronics of a Coriolis mass flow sensor, two preamplifiers provide output signals that are applied to a fixed gain amplifier and a variable gain amplifier. Outputs from the two gain amplifiers are applied to a difference stage having an output coupled to a changeover switch. The output of the fixed gain amplifier is also coupled to the changeover switch. A summing/integrating stage receives signals from the preamplifiers and is connected to a phase locked-loop followed by a phase measuring and phase adding stage having an output that provides first and second phase shifted signals. A first synchronous rectifier is clocked by the first phase shifted signal and has an output connected to a switch. A sequencer keeps the switch ON during an ON period and switches the changeover switch to the difference stage except during a short measurement period during which the sequencer activates a phase measurement. A second synchronous rectifier is clocked by the first phase shifted signal during the measurement period and by the second phase shifted signal during the ON period and has an output connected to an integrator. The input of a memory stage is connected the integrator during the measurement period. A divide stage has a dividend input, a first divisor input, and a second divisor input that are, during the ON period, connected to the integrator, the memory stage, and the phase-locked loop, respectively, and provides a signal proportional to the mass flow rate.

I claim: 
     
       1. Evaluation electronics of a Coriolis mass flow sensor having a first measuring tube vibrating at a vibration frequency and a second measuring tube vibrating at the same vibration frequency, the measuring tubes being traversed by a fluid to be measured, and a first electrodynamic transducer and a second electrodynamic transducer being positioned along at least one of the measuring tubes at a distance from each other in the direction of fluid flow, the evaluation electronics comprising: a first preamplifier for the signal from the first transducer;   a second preamplifier for the signal from the second transducer;   a first, fixed-gain amplifier for the output signal from the first preamplifier;   a second, variable-gain amplifier for the output signal from the second preamplifier;   a difference stage for the output signals from the two amplifiers;   a changeover switch whose first input is connected to the output of the difference stage, and   whose second input is connected to the output of the first amplifier;     a variable-cutoff-frequency low-pass filter whose transmission begins to decrease at the vibration frequency of the measuring tubes, and   whose input is connected to the output of the changeover switch;     a summing/integrating stage for the output signals from the two preamplifiers;   a phase-locked loop whose input is connected to the output of the summing/integrating stage, and   whose output signal is a first vibration-frequency signal;     a phase-measuring and phase-adding stage whose first input is supplied with the first vibration-frequency signal,   whose second input is supplied with the output signal from the low-pass filter,   whose first output provides a second vibration-frequency signal shifted in phase by an amount equal to the phase shift of the low-pass filter, and   whose second output provides a third vibration-frequency signal shifted in phase by an amount equal to said phase shift plus 90°;     a first synchronous rectifier clocked by the second vibration-frequency signal and fed with the output signal from the low-pass filter;   a switch having its input connected to the output of the first synchronous rectifier;   an adjustment stage for adjusting the gain of the second amplifier and having its input connected via a first integrator to the output of the switch;   a sequencer which keeps the switch ON during an ON period and the output of the changeover switch connected to the first input of the changeover switch except during a measurement period which is short compared with the ON period and during which the sequencer activates the phase measurement of the phase-measuring and phase-adding stage;   a second synchronous rectifier which is clocked by the second vibration-frequency signal during the measurement period and by the third vibration-frequency signal during the ON period, the second synchronous rectifier being supplied with the output signal from the low-pass filter;   a second integrator having its input connected to the output of the second synchronous rectifier;   a memory stage having its input connected to the output of the second integrator during the measurement period at the most; and   a divide stage wherein during the ON period a dividend input is connected to the output of the second integrator,   a first divisor input is connected to the output of the memory stage, and   a second divisor input is connected to the output of the phase-locked loop, and   whose output signal is proportional to mass flow rate.       
     
     
       2. Evaluation electronics as claimed in claim 1 wherein the low-pass filter is a switched-capacitor filter having a clock signal that is generated by the phase-locked loop, and whose clock frequency is an integral multiple of the vibration frequency. 
     
     
       3. Evaluation electronics as claimed in claim 1, comprising a measuring stage for measuring the density of the fluid whose input is supplied with the first or second vibration-frequency signal. 
     
     
       4. Evaluation electronics as claimed in claim 1 wherein the output of the second synchronous integrator is coupled directly to an analog-to-digital converter, and wherein the memory stage and the divide stage are digital circuits.